Study On Horizontal Well Borehole Stability Of Xinchang Xujiahe Formation In West Sichuan

The gas resource quantity of Xujiahe formation in west Sichuan is huge, and the exploration & exploitation prospects are good. Because the tight reservoir of Xujiahe formation, the yield is depending on the fracture development. Because of the heterogeneity of fractures,hardness of predicting natural fractures and low probability of encountering fractures, the gas development of Xujiahe formation in west Sichuan is very hard. In order to exploit the Xujiahe formation gas pool, the high angle deviated well and horizontal well should be used to increase the chance to meet the fractures. There are not many horizontal wells in Xujiahe formation, so drilling and completion technology is not mature. Now, it is urgent to study on the horizontal well borehole stability in order to provide technical support for the reasonable exploitation of gas resource of Xujiahe formation.On the base of true shear wave, this paper used the method of binomial fitting to calculate the shear wave of study area. On this basis, this paper used many ways to calculate the strength parameters and elastic parameter, such as, calibrating the sound wave frequency dispersion,selecting the lithology and quality factor and calibrating the empirical formula. On the base of experimental data, this paper established the separate relationship between the dynamic and static parameters for sandstone and mudstone.According to several kinds of well logging datas, this paper selected the ADS model to calculate the crustal stress of single well. On the basis of well logging interpretation and experimental results, the orientation of maximum horizontal principal stress is EW.Because the well track of high angle deviated well is not vertical and it is not orthogonal with the horizontal principal stress, the mechanical model of sidewall rock is needed. On the basis of this model, this paper used the Mohr-Coulomb criterion and the maximum tensile stress failure criterion to discuss the relationship between the safe mud weight and other parameters, such as, maximum horizontal principal stress,cohesion and friction angle. This paper also analyzed the sensitivity of these parameters. When the other parameters are not changed, the bigger deviation angle, the bigger safe mud weight. On the basis ofσH>σv>σh, when theσH,η,Pp increase, the collapse pressure also increase. When C andφdecrease, the collapse pressure also decrese. WhenσH,ηand Pp increase, the fracture pressure decrease. Whenσt increase, the fracture pressure also increase. It can be seen that the range of safe mud weight becomes smaller when theσH,ηand Pp increase. In this case, the sidewall rock is easy to be damaged.Finally, this paper used these research results to calculate the safe mud weight of horizontal section of X10-1H well. The calculation result coincides with the actual situation of X10-1H well and formation breakdown pressure.So, the study method about the horizontal well borehole stability of Xinchang Xujiahe formation in west Sichuan is reliable, and it can be used into the future exploration and exploitation of oil and gas resources of Xinchang Xujiahe formation in west Sichuan.